Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
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HOMESERVICE MANUALSGMC1996PICKUP C2500, 2D PICKUP, 5.0 M, AUTOMATICREPAIR AND DIAGNOSISENGINE PERFORMANCESYSTEMENGINE CONTROLS - TESTS W/CODES - GASOLINEDIAGNOSTIC TROUBLE CODE TESTSDTC P0303 - CYLINDER NO. 3 MISFIRE DETECTEDDIAGNOSTIC PROCEDURES
1996 GMC Pickup C2500, 2D Pickup, 5.0 M, Automatic
Diagnostic Procedures
1996 GMC Pickup C2500, 2D Pickup, 5.0 M, AutomaticSECTION Diagnostic Procedures
- Perform Powertrain On-Board Diagnostic (OBD) System Check. After performing OBD system check, go to next step.
- Check if any other DTCs are set. If any other DTCs are set, go to applicable DTC test. If no other DTCs are set, go to next step.
- Install scan tool. With engine idling, monitor MAP sensor voltage using scan tool. If MAP sensor voltage is greater than 4 volts, go to DTC P0108. If MAP sensor voltage is less than 4 volts, go to next step.
- Duplicate conditions required for misfire. Check if DTC is set as current for this ignition cycle. If DTC is set as current, go to next step. If DTC is not set as current, go to step 9).
- Install Spark Tester (J 26792) on spark plug wire No. 3. Disconnect fuel injector harness at intake manifold. Crank engine. Spark should jump tester gap. If spark is adequate, go to next step. If spark is not adequate, go to step 10).
- Remove spark plug from affected cylinder. Inspect spark plugs for wear, excessive air gap, cracks or fouling. Swap spark plug with a plug from another cylinder that is not experiencing misfire. Monitor misfire index using scan tool. If misfire index increases for cylinder with suspect plug, go to next step. If misfire index does not increase for cylinder with suspect plug, go to step 11).
- Perform a cylinder compression test. If a problem was found, go to step 13). If a problem was not found, go to next step.
- Diagnose injector circuit. See appropriate BASIC TESTING article. If a problem was found, go to next step. If a problem was not found, go to step 12).
- Misfire is intermittent and not currently occurring. See DIAGNOSTIC AIDS.
- Perform enhanced ignition system check. See BASIC TESTING article.
- Replace spark plug. Go to step 14).
- Repair circuit as necessary. Go to step 14).
- Repair problem as necessary. Go to next step.
- Using scan tool, select DTC, CLEAR INFO. Start and warm engine to normal operating temperature. Select DTC, SPECIFIC, then enter DTC number that was set. Operate vehicle within the conditions for setting this DTC. If scan tool indicates that this test ran and passed, go to next step. If scan tool does not indicate that this test ran and passed, repeat step 2).
- Using scan tool, select CAPTURE INFO, REVIEW INFO. If any undiagnosed DTC(s) are displayed, go to applicable DTC test.
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.